Shielding circuits for the purpose of counteracting magnetic fields in power circuits are known as are the use of compensating circuits for minimizing power circuit impedance. Additionally, it is known to use insulating devices, pipe line extensions, as well as various pipe line grounding methods, for the purpose of reducing voltages induced on gas transmission pipe lines by 60 Hz AC power transmission lines sharing a joint right-of-way as indicated by Taflove et al "Mitigation of Buried Pipe Line Voltage Due to 60 Hz AC Inductive Coupling", IEEE Transactions on Power Apparatus and Systems, Volume PAS-98, pages 1806-1823, September/October 1979.
Power utility companies, however, have been faced with the need for increased transmission system capacity while simultaneously challenged by increased public concern as to the possible effects of electric and magnetic fields produced by transmission lines. That is to say, the need for increased power transmission and distribution capacity is present along with the difficulties and expense involved in the acquisition of new transmission right-of-ways that typically produce substantial public opposition. Thus increasing power transmission capabilities over existing right-of-ways is often the only visible manner of supplying increased customer power requirements. In addition, where new transmission lines are constructed, means for minimizing associated magnetic fields must be provided to facilitate compliance with regulatory requirements.
The possible health and environmental effects, if any, of the increased electromagnetic field created by increased current at power line frequency through such transmission and distribution lines is, of course, a concern to the electric power utilities, regulatory agencies, as well as the general public. In fact, certain jurisdictions have placed limits on the magnetic field levels which can be created by a power line. Accordingly, there is a need to develop a means for allowing increased power transfer over an existing right-of-way without increasing the magnetic and electric field intensities outside such right-of-ways; similarly, a new transmission line can be added while minimizing associated magnetic fields.
The object of the disclosed exemplary embodiments is that of providing effective shielding conductors between the power circuits and the existing right-of-way edge in such a manner as to reduce such field levels, particularly electromagnetic field intensities. It is a still further object of the disclosed embodiments to accomplish such results without requiring the reconstruction of existing power lines or the modification of existing design standards in the construction of new power lines.
Briefly stated, I have discovered that the installation of a series-capacitance compensated shielding circuit located physically adjacent and parallel to a polyphase power transmission or distribution line partially cancels the electromagnetic fields created by the passage of power frequency current through the lines, thus reducing the net electromagnetic fields created by such lines outside a right-of-way. Such a passive shielding scheme can provide up to about an 80% reduction of the magnetic field at the right-of-way edges for typical existing transmission lines. My shielding scheme additionally provides a 20-40% decrease in electric field intensity at the edge of the right-of-way.